FVC is a rather ill-defined property to start with, as it is highly dependent on the scale of the system being monitored, the spatial resolution of the measuring instrument, as well as various aspects of the observation protocol. In addition, if you don't know and specify what range of errors is acceptable in your situation, then any answer will be acceptable. So, in the absence of such information, I'd suggest the answer is 42, following the method of Deep Thought!
Joke aside, please provide some background information on the nature, scope and purpose of your project, why you want to estimate FVC, what environment you are looking at, etc. The tools and methods should be selected only after your objectives are clear, including the maximum tolerable level of uncertainty.
Thanks for your answer. Indeed, what you point at worth my deep thought.
Attachment is a photo taken by SONY NEX-5R mounted on our drone. The photo contains a quadart of cornfield, with green rope circling. We hope to compare the value of FVC estimated by digital camera mounted on UAV with ground value. Our purpose is that FVC by UAV could take the place of ground value to become the validation data of satellite FVC.